Objects can be trained to follow a certain path by forces, and will continue along that path, even after the forces are removed.

A ball on a string is swung counterclockwise, so that it rolls in a circular path on a frictionless floor (shown in the diagram below). The string is released when the ball is in the position shown. Sketch the path of the ball after the string is released.

 

 

When the string is cut, the only forces acting on the ball are weight and the normal force of the floor. Since the ball is not accelerating up or down these two are equal and opposite. Thus the net force on the ball is zero, and the ball will travel in a straight line at constant speed (in the direction shown below).